This invention relates to means for flattening coiled metal strip, sheets and plates. Machines which perform this function are variously referred to as flatteners, levelers, straighteners, or roller levelers. All of these machines perform essentially the same function and operate substantially upon the same general principles. For the purposes of this application, the invention will be referred to as a roller leveler.
Although a roller leveler is equally suitable for processing sheets and plates, for illustrative purposes, the function of a roller leveler will be herein described in relation to coiled strip. Metal is formed into strip by a process known as rolling, wherein the strip is passed between a pair of work rolls of a rolling mill to reduce its cross-sectional thickness. In the process, the strip is elongated and rolling continues until the strip is reduced to the cross-sectional thickness desired. This rolling process may start with heated billets or slabs of metal, wherein the metal is rolled at a very high temperature, or it may start with previously rolled strip wherein the strip is passed between work rolls in the cold state. In either event, when the strip exits from the mill it may be convolutely wrapped to form a so-called coil. When the coil has been formed, curvature of the coil tends to stay with the strip when it is necessary to uncoil the strip for further processing. Thus, the primary problem with strip coming off of a coil is the curvature which remains with the strip and which varies throughout the entire length of the coil as a function of the radius of any particular portion of the strip while in the coil. Accordingly, the outer wrap of the coil will have less curvature than an inner wrap. To remove this variable curvature in the strip is one of the purposes of a roller leveler. It is necessary to remove this curvature so that the strip may be cut accurately and rendered suitable for other manufacturing operations, such as punching, drawing, forming and the like. It is well established that the flatter the strip is prior to a subsequent manufacturing operation, the more accurate and satisfactory will be the end product of that operation. Thus, even where portions of steel strip are deep drawn, they do not draw as satisfactorily if the strip initially is not substantially flat before the draw.
The theory of operation of a roller leveler is quite simple in principle. It is to take an unknown problem and convert it into a known problem for which there is a known solution. By way of example, when the strip is taken from the coil it is not known what the particular degree of set is in any particular portion of the coil. Accordingly, the strip is passed through a combination of rollers which flex the strip a predetermined amount first in a given direction and then a predetermined amount in the opposite direction. Reverse flexing the strip in this manner by lesser and lesser amounts will eventually remove all curvature from it, irrespective of the degree of curvature set in the strip prior to entering the roller leveler.
In addition to strip curvature, other unwanted properties are sometimes impressed upon the strip during hot and/or cold rolling which render the problem of flattening strip much more complex. In order to reduce cross-sectional thickness of the strip during rolling, it is necessary to force the strip between rolls under tremendous pressure whereby the strip essentially becomes a wedge which tends to separate the rolls. The force of roll separation is dependent upon the physical properties of the strip including width, thickness, hardness, temperature, yield strengh, and amount of reduction being attempted during the pass of the strip between the rolls. If the work rolls are not sufficiently supported by so-called back-up rolls it is possible for the strip to actually cause the work rolls to bend at their centers, wherein the resultant strip cross-sectional shape is thicker in the middle than at the edges. Strip rolled with thicker center portions indicates that greater pressure has been applied to the edges of the strip than at the center, thereby causing the edges to elongate at a greater rate than the center of the strip. Because this excess metal on the edges must go somewhere, but is restrained by the center, the result usually is a product having what is referred to as edge waves. In other words, the center of the strip is relatively flat longitudinally but the edges of the strip are sinusoidal. Strip rolled with edge waves is usually not saleable.
Just the opposite may occur during rolling of strip, wherein the rolls may be so reinforced, or may be so contoured, that they resist or otherwise offset the wedge effect of the strip. However, if the rolls are over compensated against roll bending, the resultant is strip that is rolled thinner in the center than at the edges. In this circumstance, the center of the strip tends to become elongated, producing a condition sometimes referred to as "oil canning". By this is meant that the elongated center portion of the strip compensates for this elongation by bulging either up or down. The result is strip that can literally be snapped up and down like the bottom of an oil can because of the stresses set up by this localized elongation.
Essentially, therefore, a strip coming to a roller leveler from a rolling mill could conceivably have several basic defects. The strip could have a curvature set because it was formed into a coil, the strip could have edge waves because its center was rolled thicker than its edges, the strip could have oil canning because its center was rolled thinner than its edges, or the strip could have combinations of these defects.
It was discovered long before the subject invention that roller levelers, in addition to taking curvature out of coiled strip, could also remove the edge waves and/or the oil canning condition of the strip by skillful manipulation of the work rollers. On the other hand, if the strip came from the rolling mill fairly flat, an improperly operated roller leveler could create edge waving and/or oil canning in the strip. Thus, it was possible for the strip to exit the roller leveler in worse condition than it entered.
In order to avoid reducing the strip to poorer condition than when it was received, and at the same time correct what defects had been rolled into the strip from the mill, it has heretofore been necessary for an operator to continuously monitor and to adjust the work rollers of a roller leveler during the entire pass of the coil through the roller leveler. Obtaining a high quality of strip flatness from a prior art roller leveler is an art which can only be learned by an operator after many years of experience. Thus, it has been known in the prior art to bend the work rollers of a roller leveler to correct edge wave, oil canning and curvature. This is done by manipulating the work rollers of roller levelers. In the simplest form, a roller leveler comprises an upper work roller and two lower work rollers. However, in a practical industrial roller leveler the number of rollers are a matter of choice depending on the particular type of work being performed, and roller levelers having as many as twenty-nine rollers are known. It is also known that the more aggravated the condition of the non-flatness of the strip, the more rollers are required to bring the strip back to a flat condition. Particularly is this so in correcting edge waving and oil canning.
By way of general organization, a prior art roller leveler may include opposed upper and lower banks of work rollers and their associated back-up rolls. The upper bank of work rollers extends from side to side of the frame of the roller leveler and are positioned in parallel one behind the other from front to rear of the frame. The lower bank of work rollers also extends from side to side and from front to rear of the roller leveler frame and are parallel to the upper work rollers. However, the lower work rollers are offset so that an upper work roller may be brought substantially into tangential or nesting contact with a pair of lower work rollers. The spacing between the upper work roller and a pair of lower work rollers permits passage of the strip over a lower work roller, under the adjacent upper work roller and then over the next lower work roller. This spacing is referred to in the trade alternatively as the gap or plunge of the rollers. The more an upper work roller is plunged between a pair of lower work rollers, the greater is the so-called plunge which has been applied to the rollers. Conversely, the greater the plunge, the smaller is the resultant gap. This adjustment of rollers has been accomplished in the prior art with hydraulic jacks, mechanical screw jacks, wedges and the like.
In prior art roller levelers, each bank of work rollers can be shifted vertically up or down as a unit to increase or lessen the plunge between the upper and lower work rollers. Customarily, the upper and lower banks of rollers can also be tilted as a unit to provide decreasing plunge between the upper and lower work rollers from front to back. Thus, the flex of the strip at the entrance to the roller leveler may be relatively severe but this flexing will become less and less pronounced as the strip progresses between the work rollers from entrance to exit of the roller leveler.
To prevent the work rollers from bending due to the separating force of the strip while being flexed sinusoidally between upper and lower work rollers, relatively short back-up rolls are evenly spaced across the span of each work roller to prevent unwanted bending of an individual work roller. Each work roller may have as many as five or more small back-up rolls in tangential contact therewith. Corresponding back-up rolls from work roller to work roller may be in alignment from front to rear of the roller leveler and this alignment is referred to as a flight of back-up rolls. Thus, if each work roller has five supporting back-up rolls extending from side to side of the work roller, there would be five flights of back-up rolls extending from front to rear of the roller leveler.
In the prior art, each flight of back-up rolls is usually mounted on a massive beam, also extending from front to rear of the roller leveler frame. It is known for the beams to be moveable up or down but not to be tiltable. Only the entire bank of either upper or lower rolls are tiltable. Thus, by manipulating the back-up roll beams, which can be done by mechanical screw jacks, hydraulic cylinders, wedges and many other mechanical devices, the relative position of flights of back-up rolls may be adjusted within limits with respect to the work rollers. An experienced operator observing strip edge waving, oil canning or both, can, by manipulating the back-up roll beams up or down, bend the work rollers to remove the edge wave or the oil canning. However, it is important to emphasize that all of these adjustments in prior art roller levelers are manual and, as already stated, require great skill of the operator.
It is not unusual in roller levelers for the strip work product to exert a total separation force against the work rolls of approximately two million pounds. Thus, in the case of a roller leveler having five sets of back-up rolls per work roller, the separation force would be two hundred tons per flight of back-up rolls. To perform a good job of flattening the strip, the operator might be required to set the desired gap between rollers within 0.001 to 0.002 inches. However, the separating force of the strip between the rollers could cause, even under normal operation, a roller deflection of 0.030 to 0.120 inches. Furthermore, in addition to causing roller bending, the separating forces of the strip also cause the frame itself to bend and to stretch. Essentially, the inadequacy of the prior art roller levelers resides, therefore, in the fact that with the sides of the frame stretching, the crown of the frame bending, the base of the frame bending, and the rollers bending, there is no point of reference on the frame from which to maintain a predetermined gap between the rollers. With a prior art roller leveler, it would be ineffective to place a sensor on the crown of the frame to detect work roll deflection since it is conceivable that at the same time that the work roll is deflecting 0.030 inches the crown of the roller leveler is also deflecting 0.030 inches, whereby the sensor would read no deflection whatsoever. In like manner, if the sensor were mounted on the side of the roller leveler frame, wedging apart of the rollers by the strip could not be accurately measured because of the stretch in the sides of the frame.
The foregoing is a brief summary of the state of the prior art. No automation, heretofore, has been accomplished with roller levelers because the roller leveler is in a state of dynamic change during a leveling operation at which time virtually all parts of the roller leveler are being subjected to stresses and strains of indeterminate magnitude and duration, uncontrolled and continuously varying. The only means available to the prior art to cope with the above uncontrolled variables in roller leveler mechanisms is to make manual adjustments, solely at the discretion of the operator. The end product, therefore, is a direct function of the skill of the operator to cope with all of the variables of the roller leveler under stress and strain. Successful automation of the operation of a roller leveler prior to the subject invention has, within the applicant's knowledge, never been accomplished.